Email body sometimes uses multiple columns

I’m using a fairly simple shortcode to compose a newsletter: [sp-recent-posts posts=100 columns=1 displayimages=false]

When viewed in Thunderbird, it sometimes uses more than 1 column (up to 12 columns sometimes) even if columns=1 is set. Also, it’s ignoring displayimages=false. It’s still sending thumbnails next to each item.

Is there a way to send a simpler message that has a bullet point for each item with only the post title (hyperlinked)?

Thank you